The main process for the conventional synthesis of acrylic acid uses a catalytic reaction of propylene with a mixture containing oxygen. This reaction is generally performed in the vapor phase, and usually in two steps:                the first step effects the substantially quantitative oxidation of propylene to an acrolein-rich mixture, in which acrylic acid is in minor amount,                    the second step effects the selective oxidation of acrolein to acrylic acid.                        
The reaction conditions for these two steps, performed in two reactors in series, are different and require catalysts that are suitable for the reaction. It is not necessary to isolate the acrolein during this two-step process. The reaction may also be performed in a single reactor, but, in this case, it is necessary to separate out and recycle large amounts of acrolein at the oxidation step.
It has been known for a long time that acrolein may be obtained from glycerol. Glycerol (also known as glycerine) is obtained from the methanolysis of plant oils at the same time as methyl esters, which are themselves used especially as fuels or combustibles in diesel and domestic fuel oil. It is a natural product that has an “environmentally friendly” image, is available in large quantity and may be stored and transported without difficulty. Numerous studies have been devoted to the financial upgrading of glycerol according to its degree of purity, and the dehydration of glycerol to acrolein is one of the routes envisioned.
The reaction involved for obtaining acrolein from glycerol is:CH2OH—CHOH—CH2OH⇄CH2═CH—CHO+2H2O
As a general rule, the hydration reaction is favored at low temperatures, and the dehydration reaction is favored at high temperatures. To obtain acrolein, it is thus necessary to use a sufficient temperature, and/or partial vacuum to shift the reaction. The reaction may be performed in the liquid phase or in the gas phase. This type of reaction is known to be catalyzed by acids. Various processes for synthesizing acrolein from glycerol are described in the literature: FR 695 931; U.S. Pat. No. 2,558,520; WO 99/05085; U.S. Pat. No. 5,387,720.
The reaction for the dehydration of glycerol to acrolein is generally accompanied by side reactions leading to the formation of by-products such as hydroxypropanone, propanaldehyde, acetaldehyde, acetone, adducts of acrolein with glycerol, glycerol polycondensation products, cyclic glycerol ethers.
In international patent application WO 2005/073160, the reaction product resulting from the gas-phase dehydration reaction of glycerol is subjected to a subsequent gas-phase oxidation step to obtain acrylic acid. This two-step process is performed with pure glycerol or concentrated aqueous solutions comprising more than 50% by weight of glycerol. Acrylic acid yields of between 55% and 65% are obtained using an alumina-based catalyst impregnated with phosphoric acid and silica for the first dehydration step, and an alumina-supported Mo—V—W—Cu—O mixed oxide for the second oxidation step. The process is performed in two reactors in series, the oxygen being able to be added easily to the gaseous mixture feeding the second reactor. The process may also be performed in a single reactor containing either the two stacked catalysts defining two reaction zones, or the two catalysts as a mixture, this configuration having the drawback of more rapidly deactivating one of the two catalysts, which makes maintenance of the process difficult.